No need of a script..., just a calculation into the option of auto-enter a value for the field: Members::Rate Paid
Members::Fee Paid = 100 ; Get ( CurrentDate )
You can also put a field name in place of the constant value: 100 so that if your dues change, you don't have to edit this expression to update it:
Members::Fee Paid = Table:: DuesAmt ; Get ( CurrentDate )
"Table:: DuesAmt" would be a field in a related table (use the X operator so all records match your main table). Now you can set up a layout with this DuesAmt field on it and you can update your required Dues amounts without redesigning your calculated auto-enter expression.